Arctic melt versus Antarctic freeze: Is Antarctica warming or not?

During the short austral summer, much of the sea ice surrounding Antarctica melts, often leaving only the large, tabular icebergs. Credit: NSIDC courtesy Andi Pfaffling

September 2012 was a record-setting month for both of Earth’s poles, but for different reasons: sea ice in the Arctic fell to a record low minimum extent after a summer of melting, while Antarctic sea ice froze to a record high extent during the South Pole winter. Is record Antarctic sea ice canceling out the losses in Arctic ice? And does the record in the south mean that Antarctica is not warming?

Warming at the South Pole

Polar scientists say no on both counts. Ted Scambos, lead scientist at NSIDC, said, “These systems are not directly connected, and they certainly don’t offset each other. The climate and ocean processes that control summer Arctic ice extent are completely different from the ones that drive the Antarctic.” Data records indicate warming at both poles, but Antarctica’s geography is forcing warming to show in different ways than it does in the Arctic. “Antarctica’s trend is not nearly as large or as clear as the Arctic’s,” Scambos said.

Antarctica is a vast and largely ice-bound landmass, and the effects of warming are more complicated for scientists to sort out. The East Antarctic Ice Sheet has remained fairly stable, but air temperatures over the Antarctic Peninsula and the West Antarctic Ice Sheet are rising. Along the Peninsula, warmer air has increased surface melting, and several large ice shelves along the Peninsula have disintegrated dramatically after particularly warm summer periods.

Although Antarctica remains a cold place, this data image illustrates warming across the continent. Red represents areas where temperatures, measured in degrees Celsius per decade, have increased the most during the last 50 years, while dark blue represents areas with a lesser degree of warming. West Antarctica and the Antarctic Peninsula, the craggy finger of land jutting out from the continent, have experienced the most warming. Credit: NASA/GSFC Scientific Visualization Studio

In spite of warmer air, the climate of rest of the West Antarctic Ice Sheet remains colder than the Peninsula, so the rise in temperatures has not led to much summertime melting. There, the main culprit is warmer ocean currents. New ocean circulation patterns are bringing much warmer water to the ice edges along the coast. This is rapidly thinning the thickest glaciers at their base, and causing them to speed up. Consequently, glaciers in West Antarctica are also losing ice, in much larger amounts than glaciers along the Peninsula.

Wind, weather, and ozone

Yet warming is only part of the story. Changes in Antarctica’s winter sea ice are due more to changes in the winds that encircle the continent and, strangely enough, in the ozone layer above it. “The winds that blow around the continent have gotten stronger,” Scambos said. “And an added effect is the ozone hole, which also changes winds in the far south.” This seasonal hole forms over Antarctica each winter. Normally, ozone absorbs sunlight and warms the atmosphere. The current lack of ozone leaves a much colder air mass above Antarctica. While this may sound ideal for ice formation, it can sometimes have the opposite effect on sea ice. When warmer air from the north mixes with stubbornly cold air over Antarctica, the resulting turbulence generates stronger winds. During winter, stronger winds often blow the ice northward, away from the continent, creating a larger overall extent. These winds shifts are also behind the new patterns of ocean circulation that is warming glaciers at the edges of the West Antarctic Ice Sheet.

For now, however, most of Antarctica remains a very cold place, and it has not yet reached the critical threshold beyond which melting further amplifies changes. “Both the Arctic and Antarctic systems are being driven in new directions by human-caused changes. At this point, the side effects for the Antarctic are outweighing the effect of warming,” Scambos said.